Treating aluminum surfaces



Patented Apr. 7, 1942 TREATING ALUMINUM SURFACES 1 Charles J. Slunder,Oakmont, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa.,a corporation of Pennsylvania No Drawing.

6 Claims.

This invention relates to the production of oxide-coated aluminumarticles having improved Application December 23, 1937, Serial No.181,312

surface characteristics, and more particularly to the production ofoxide-coated aluminumarticles or parts for use in refrigerators andhaving improved properties.

In the manufacture of various articles of aluminum, it is the practicein some cases to form artificially on the surface thereof I a protectivecoating which is composed substantially of aluminum oxide and generallyknown in the art as an oxide coating. While aluminum having its surfacethus coated has been quite successful commercially, as evidenced by itsuse'in the various arts, for certain applications the inherentproperties of the oxide coatings are such that the oxide-coatcdarticlesare not wholly satisfactory. 4

For instance, oxide coatings on aluminum surfaces have been found topossess a slight odor, sometimes referred to as a chalky or earthy"odor, which is generally not objectionable and is, in fact, usuallyunnoticed, but which is sometimes considered undesirable, especially inoxidecoated aluminum articles which in use are normally confined withina relatively small air space, such as articles or parts used inrefrigerators.

Another peculiarity of oxide-coated aluminum surfaces is noted when theyare used in contact with freezing water, such as theice-cube trays usedin mechanical household refrigerators and the aluminum grids orseparators used in them Considerable difficulty and annoyance have beenexperienced in removing the ice from oxidecoated aluminum ice-cube traysbecause of the tendency of the ice formed in contact with an oxidecoating to stick or adhere to the surface. This difficulty is overcometo some extent by the provision of levers or other mechanical devicesattached to the containers which in use do not reduce the adherence ofthe ice to the container, but merely afford increased mechanicaladvantage to permit application of more force in removing the ice.Attempts have also been made to overcome this difficulty by reducing theadherence of the ice to the oxide-coated surface by applying molten Waxto the surface of the oxide coating. This solution of the problem is notentirely satisfactory, however, because the covering of the oxidecoating with wax and the removal of the excess wax is an operation whichadds materially to the cost of production of the article.

It is an object of this invention to provide an article having anoxide-coated aluminum surface which is substantially free of odor. It isanother object of this invention to provide an article having anoxide-coated aluminum surface which has a relatively low adherence toice formed in contact therewith.

It is a primary object of the invention to provide a simple andeconomical process for the production of oxide-coated aluminum articlessuch as refrigerator parts, ice-cube trays and the like, which may besubstantially free of odor and which present a relatively low adherenceto ice formed in contacttherewith.

, Further objects and advantages of this invention will be apparentfromthe following description. 1

In the copending application for United States patent, Serial 'No.181,311, filed December 23,-"

1937, by Charles J. Slunder, there is described and claimed a method forthe production of oxide-coated aluminum surfaces which'are free from thecharacteristic odor of oxide coatings and have relatively low adherenceto ice formed in contact therewith, in which the oxide-coated aluminumsurface is treated with an aqueous solution of a sulfuric reactionproduct.

The present invention is predicatedon the discovery that theeffectiveness of a solution of a sulfuric reaction product in reducingthe adherence of ice to an oxide-coated aluminum surface can besubstantially increased by incorporating in the solution an aqueous waxemulsion. Furthermore, I have found that the sulfuric reaction productsact to prevent the breaking of the wax emulsion which ordinarily occurswhen plain solutions of such wax emulsions are used in the treatment ofoxide-coated aluminum surfaces. Thus, by the method of this invention ithas been found possible to add the sulfuric reaction product and the waxemulsion to the hot water bath ordinarily used for sealing porous andadsorbent oxide coatings, and to effect the impregnation of the oxidecoating with wax and sulfuric reaction product without increasing thenumber of steps used in the production of ordi-' ing" is used herein andin the appended claims to describe a coating consisting substantially ofaluminum oxide artificially formed on an aluminum surface by chemicaltreatment, with or without the use of externally-applied electricalenergy, but does not include the thin film of aluminum oxide which isnaturally formed on an aluminum surface upon exposure to air. For thepurpose of the invention, the oxide coating may be formed on thealuminum surface by any one of several methods, for example, by treatingthe aluminum article electrolytically as the anode in a solution ofsulfuric acid, chromic acid, or oxalic acid, or by treating it, bysimple immersion or otherwise, in a hot solution of an alkali carbonatecontaining a 'dichromate.

In forming the oxide coating on aluminum refrigerator parts, it ispreferred to treat the aluminum surface electrolytically as anode in anaqueous solution of sulfuric acid. The concentration of the acid mayrange from about 1 to 10 per cent, but I prefer a concentration between15 and 25 per cent. When a solution containing 15 to 18 per cent ofsulfuric acid is used at a temperature of about 68 to 74 F., a currentdensity of about 12 to 13 amperes per square foot is impressed upon theanode, and the action of the electrolyte on the aluminum is allowed tocontinue for about 10 to 30 minutes.

When using a-solution of sodium carbonate and potassium dichromate toproduce an oxide coating on an aluminum article, the piece to be coatedis preferably immersed in a solution containing about 0.1 to 1.0 percent of potassium dichromate and 0.5 to 6.0 per cent of sodiumcarbonate. about 2.0 per cent sodium carbonate and 0.5 per cent ofpotassium dichromate at or near the boiling temperature.

After a suitable oxide coating has been formed on the aluminum surface,it is rendered substantially free from its characteristic odor, and atthe same time is provided with the property of low adherence to iceformed in contact with it, by subjecting the oxide coating to the actionof an aqueous solution of a sulfuric reaction product containing anemulsified waxin suspension. The sulfuric reaction product may be usedin amounts-as small as about 0.1 gram per liter and in amounts .up tothe solubility limit of the compound. For most purposes it has beenfound desirable to use the sulfuric reaction products in concentrationsof 0.5 to grams per liter. The wax used may be any of the water emulsionwaxes and is preferably introduced into the solution of the sulfuricreaction product in the form of an aqueous wax emulsion containing about5 per cent to 30 per cent of wax. The amount of wax emulsion which maybe used will vary over a relatively wide range of concentrations,depending on the concentration of the sulfuric reaction product presentin the solution. I have found that in order to maintain the emulsifiedwax in suspension, particularly when the treating solution is to be usedhot, it is desirable that the wax be added to the solution of sulfuricreaction product in amounts corresponding to not more than about 5 partsof wax to each part of sulfuric reaction product in solution. In thepreferred practice, an aqueous wax emulsion containing about to 25 percent of wax is used in sumcient quantity to supply 0.25 to 2.5 parts ofwax to each part of sulfuric reac- I prefer to use a solution containing,ass

be present in amounts in excess of the-above proportions up toits-solubility limit.

The solution of the sulfuric reaction product containing the'waxemulsion may be used at various temperatures, but it is preferred to useit at temperatures above about 80 C. The treatment is carried outpreferably by immersing the oxide-coated article in the treatingsolution at or near boiling for a period of about minutes.

The sulfuric reaction products contemplated herein and in the appendedclaims are the watersoluble inorganic or organic salts of the productsproduced by the action of sulfuric acid or its equivalent upon aliphaticalcohols of high molecular weight. Compounds of this nature which havebeen found to produce satisfactory results amine. According to theconditions of the reaction, either a sulfuric acid ester is obtained byreaction of the hydroxyl group, of the alcohol with the sulfuric acid,or aliphatic sulfonic acids, or mixtures of these two types of compoundsmay be obtained. The use of either or both of these types of compoundsis contemplated as falling within the scope of this invention. Thewatersoluble alkali metal salts of these compounds tion product. Thesulfuric reaction product may have been found particularly suitable tothe practice of this invention, the sodium salt of the sulfated lauryl,cetyl and stearic alcohols being preferred.

The waxes which have been found suitable for the purposes of thisinvention are the waxes which may be made to form a substantially stableemulsion with water. Typical examples of such waxes are carnauba wax,paraflin wax, certain types of synthetic waxes, and the like. Any of theusual emulsifying agents may be used, such as alkali metal, ammonium, orethanolamine soaps, and the like. Aqueous wax emulsions suitable for thepurposes of this invention may be formed in various ways known in theart. One emulsion which has been found particularly suitable is anaqueous emulsion of carnauba wax containing about 25 per cent of wax.

In the following example there is set forth a comparison of two aluminumice-cube trays, one of which was treated according to the method of thepresent invention, the other of which was providedwith an aluminum oxidecoating and subsequently sealed without the use of the compoundscontemplated by this invention. In forming these two trays, .they weretreated as anodes in a sulfuric acid electrolyte containing 15 per centsulfuric acid at a current density of about 12 amperes per square footfor a period of about 20 minutes at a temperature of about 70 F; One ofthe trays was then immersed in a boiling aqueous emulsion prepared bythe addition of 5 grams of an aqueous emulsion of carnauba waxcontaining 25 per cent by weight of wax and 1.35 grams of a sodium saltof sulfated stearic alcohol to each liter of water, for a period ofabout 20 minutes. The other tray was treated for a like period inboiling water.' The trays were then the group consisting of thesubstantially, alkali washed and dried, and it was noted that thecharacteristic odor of the oxide coating was absent from the traytreated with the wax emulsion and sulfated stearic alcohol composition.Both trays were then filled with water and ice was frozen in them. Thetrays were then tested by a standard test to determine the pull requiredto remove the ice from the trays. In this test, it was found that theoxide-coated surface treated with the emulsion required a pullof 65pounds to remove the ice therefrom, whereas 280 pounds were required forthe removal of ice from the tray having the oxide coating which had notbeen treated with the composition of this invention.

The invention is not to be considered as limited 1 to the coating ofrefrigerator parts, such as icecube trays, grids, and the like, but isadaptable to use in connection with the coating of any aluminum articlein which the properties of a substantially odor-free oxide coating andrelatively low adherence to ice formed in contact therewith are desiredsingly or in combination.

The term aluminum as used herein and in the appended claims is intendedto include both aluminum in its various grades of purity and thealuminum base alloys.

The term odor-free as used in this specification and in the appendedclaims means free from the characteristic odor of oxide coatings.

I claim: 3

1. An aluminum article provided on its surface with an adherent oxidecoating, said coating having been impregnated with an aqueous mixturecontaining a solution of a material selected from the group consistingof the substantially 3.

alkali neutralized sulfonated and sulfated fatty alcohols and thesubstantially organic base neutralized sulfonated and sulfated fattyalcohols, and a wax emulsion.

2. An oxide-coated aluminum ice-cube tray having a relatively lowadherence to ice formed in contact with its surface, said surfacecomprising an adherent oxide coating, said coating having beenimpregnated with a mixture containing an aqueous solution of a materialselected from i neutralized sulfonated and sulfated fatty alcohols andthe substantially organic base neutralized sulfonated and sulfated fattyalcohols, and a wax emulsion.

3. A method of treating an oxide-coated aluminum surface comprisingsubjecting said surface to the action of a mixtureof an aqueous solutionof a material selected from the group consisting of the substantiallyalkali neutralized sulfonated and sulfated fatty alcohols and thesubstantially organic base neutralized sulfonated and sulfated fattyalcohols, and a wax emulsion.

4. A method of treating an oxide-coated aluminum surface comprisingsubjecting said surface to the action of a hot mixture of an aqueoussolution of a material selected from the group consisting of thesubstantially alkali neutralized sulfonated and sulfated fatty alcoholsand the substantially organic base neutralized sulfonated 'and sulfatedfatty alcohols, and a wax emulsion.

5. A method of treating an oxide-coated aluminum surface comprisingsubjecting said surface to the action of an aqueous mixture containing awax emulsion, and a material selected from the group consisting of thesubstantially alkali neutralized sulfonated and sulfated fatty alcoholsand the substantially organic base neutralized sulfonated and sulfatedfatty alcohols in amounts corresponding to not more than about fiveparts of Wax to each part of said substantially neutralized sulfonatedand sulfated fatty alcohol.

6. A method of treating an oxide-coated aluminum surface comprisingsubjecting said surface to the action of an aqueous mixture containing awax emulsion, and a material selected from the group consisting of thesubstantially alkali neutralized sulfonated and sulfated fatty alcoholsand the substantially organic base neutralized sulfonated and sulfatedfatty alcohols in amounts corresponding to about 0.25 to 2.5 parts ofwax to'each part of said substantially neutralized sulfonated andsulfated fatty alcohol.

CHARLES J. SLUNDER.

